This application claims the priority of German Application No. 100 41 803.1 filed Aug. 25, 2000, which is incorporated herein by reference.
This invention relates to an oil scraper piston ring for internal-combustion engines. The piston ring has an outer circumferential surface adapted to slide on the inner surface of the cylinder and further has, along its circumference, recesses which reduce the radial wall thickness. Each recess extends from the inner circumferential ring surface radially in the direction of the outer circumferential ring surface.
German published patent application 198 08 483 describes an oil scraper piston ring for internal-combustion engines. The piston ring has circumferentially distributed slots which extend axially through the entire ring cross section and form oil outflow openings and which, at the same time, increase the flexibility of the ring.
U.S. Pat. No. 3,378,268 describes a multi-part oil scraper piston ring which has two webs engaging the cylinder wall and which further has circumferentially distributed recesses which reduce the radial wall thickness and which extend from the inner circumferential ring surface inside toward the external circumferential ring surface. The recesses are triangular in cross section so that, similarly to the earlier-noted German published application 198 08 483, these recesses are essentially slots. The slots which taper as viewed from the cross section serve to accelerate the outflow of oil. The reference makes no mention of an increase of the ring flexibility.
A construction similar to the earlier-noted German published application 198 08 483 is described in German published application 11 01 070. According to this reference cuts are provided that extend from the inner circumference of the ring in the direction of its outer circumferential surface, whereby the moment of ring resistance may be limited.
It is disadvantage of the slit-like recesses disclosed in the prior art that garland formation during the winding of the sheet metal oil scraper piston rings is a potential risk.
The decisive parameter of the shape-conforming capability of oil scraper piston rings is the radial geometrical moment of inertia in addition to the tangential force. Such a moment of inertia depends from the radial wall thickness in the third power. In case the wall thickness is, for example, excessively reduced by providing too many slits, a simple installation of the piston ring into the engine cannot be performed.
It is an object of the invention to provide an improved oil scraper piston ring of the above-outlined type, whose ease of installation is preserved while its flexibility is optimized and its manufacturing costs are lowered, and furthermore, a reduction of the tension in the region of the smallest radial wall thickness can be achieved. It is a further object to provide an oil scraper piston ring, particularly the type which includes an expander spring, having a high degree of shape-conforming capability.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the oil scraper piston ring includes an inner circumferential surface; an outer circumferential surface adapted to contact and slide along an inner wall of an engine cylinder; a radial wall thickness defined by a radial distance between the inner and outer circumferential surfaces; and a plurality of circumferentially distributed recesses each extending radially from the inner circumferential surface toward the outer circumferential surface to reduce the radial wall thickness. Each recess has a first recess region open at the inner circumferential surface and extending radially toward the outer circumferential surface and a second recess region merging into the first recess region. The two recess regions have different shapes, and the area of the second recess region is larger than the area of the first recess region.
By the purposeful geometric arrangement and configuration of the recesses according to the invention the flexibility of the oil scraper piston ring, its shape-conforming capability and sealing effect at the flanks are significantly increased. A risk of garland formation caused by the presence of slots is minimized and a reduction of the tension in the region of the smallest radial wall thickness is achieved.
According to a preferred embodiment of the invention the recesses provided in the piston ring have two recess regions which together form a generally mushroom-shaped outline. The longitudinal axes of the two regions are oriented approximately perpendicularly to one another and intersect approximately at the half height point of the larger recess region. The circumferentially measured width of the smaller region, extending from the inner circumferential ring surface and having parallel edges and is thus generally slot-shaped, is less than the circumferentially measured width of the larger region into which the smaller region merges.
The width of the smaller recess region is preferably smaller than the width (clearance) of the ring gap defined between the two end faces of the piston ring. This feature allows that the oil scraper piston rings may be sorted for further processing by gap sizes without the risks that during the sorting operation, the ring becomes seated on the sorting shaft with the smaller recess region.